Until now, scientists had thought that H. erectus adults had tall, slender bodies, possibly as adaptations to living in a hot, tropical environment and for endurance running.

H. erectus females were assumed to have had narrow hips and relatively small birth canals that could allow the passage of only small-brained infants.

But such theories were largely based on measurements of the pelvis of "Turkana Boy," a 1.5-million-year-old juvenile male H. erectus fossil discovered in Kenya in 1984.

Experts have always considered pelvis reconstructions based on Turkana Boy rough drafts. Aside from the problems associated with extrapolating from male to female pelvis sizes, the Turkana Boy specimen was very fragmented.

"Unfortunately, the parts that are missing are crucial to the reconstruction," Simpson said.

Gorilla-like Differences

Dan Lieberman is an anthropologist at Harvard University who was also not involved in the study.

"This [new] pelvis is a nice addition to the fossil record," Lieberman said. But he added that the discovery "raises many more questions than it answers."

For example, it was known that H. erectus was probably sexually dimorphic, meaning males and females had different body sizes and shapes. The new fossil suggests the size difference may have been akin to that of gorillas, where males are much bigger than females—an idea Lieberman isn't sold on.

"For this pelvis to be a female H. erectus, then we need to accommodate really considerable sexual dimorphism with very tall and narrow males and very short and wide females," Lieberman said. "I need to be convinced."

There is also the question of how shorter and stouter H. erectus females would have managed to shed heat in the hot African climate.

Long, slim bodies shed heat better than short, stout bodies. A slender frame has more surface area relative to body volume, allowing sweat to cool the body more effectively.

"We know from many other skeletal features that H. erectus were active hominins in the midday sun in hot East Africa," Lieberman said. "If so, were males but not females adapted to heat?"

Chris Ruff, an anatomist at Johns Hopkins University who was not involved in the study, called the discovery a "landmark" that could set a new standard for how complete a hominin fossil can be.

But Ruff wonders if the pelvis might not have belonged to another hominin that lived at the same time as H. erectus.

"They state that only H. erectus remains have been found in this area for this time period, [but] there were other things around in East Africa, such as Homo habilis, which lived until 1.4 million years ago at least," he said.